Wireless communication systems, such as radio frequency (RF) communication systems, include an antenna for receiving and transmitting RF signals. Antennas can be fabricated according to a number of different designs and sizes. Certain types and sizes of antennas provide better transmission and reception for certain types of communications. Antennas for RF communication systems are often affixed to military and civilian naval, ground and airborne vehicles, personnel or platforms for relaying messages and data.
When line-of-sight RF communications are not possible for terrestrial communications systems, communications are performed using infrastructure-based systems or using beyond line-of-sight HF communication systems. An infrastructure-based systems, such as, satellite systems, or man-made infrastructure systems, such as, wire line systems, are expensive. These systems require equipment to be placed in or above the atmosphere or on land to assist the communication. The infrastructure-based systems must be available in the particular area for which communications are pending. Also, the infrastructure can be vulnerable to enemy destruction.
Beyond line-of-sight communications systems rely on the transmission of radio waves to the ionosphere and the bouncing of those radio waves off the ionosphere to a recipient. The range for the communications is increased as compared to line-of-sight terrestrial wireless communication systems by bouncing RF signals off the ionosphere.
An example of an incident communication or beyond line-of-sight system is a high frequency (HF) communications system. HF communication systems generally utilize the 3-33 megahertz (MHz) frequency range and require large antennas due to the larger wavelength associated with RF signal. One example of an incident HF communication system, the HF near-vertical incident skywave (NVIS) radio, supports a range of communication across a range of approximately 5 to 100 miles. Examples of HF NVIS for military communication system programs include the U.S. Army War Fighter Information Network—Terrestrial (WIN-T) system, the U.K. Ground Recognized Air Picture Initial Operating Capability (GRAP IOC) system, the Taiwan Advanced Technical Data Link (TATDL) system and the HF Nap-of-the-Earth (NOE) ARC220 radio for the U.S. Army.
With reference to FIG. 1, a graph 19 shows the operational effectiveness as a function of range for several antenna types used with the conventional AN/PRC-74 radio in mountainous and varied terrain including jungles in Thailand. A Y axis 21 represents a ratio of messages received over the total messages transmitted and an X axis 23 represents range in miles.
Graph 20 is an approximation of FIG. M-15 from Army Field Manual FM241-18. Graph 20 shows message reliability for three types of HF antennas: a dipole antenna, a slant antenna, and a whip antenna.
An HF whip antenna is generally an omnidirectional antenna that can be easily and quickly assembled and directed. It is typically lightweight and easy to carry but has limited range with conventional radios. Whip antennas are often comprised of a somewhat flexible conductor mounted at one end to a radio housing, a vehicle or other platform.
An HF dipole antenna generally requires two or more supports. It utilizes a conductor or wire connected between two or more supports or poles. Although assembly is relatively easy and extended ranges are possible with dipole antennas, operation on-the-move is not available. Since HF dipole antennas are often greater than 100 feet long.
A HF slant wire antenna generally requires only a single vertical support. A conductor or wire is connected to the single vertical support and the ground. Slant wire antennas can be deployed in a clearing using a tent pole and a length of conductor or wire. The slant wire antenna, similar to the dipole antenna, can be longer than 100 yards and provides an effective vertical takeoff angle that can be used to bounce the RF signals off the ionosphere.
Both the dipole antenna and slant wire antenna require greater setup time than the whip antenna and utilize a substantially longer conductor. Large setup times (from over one to two minutes) are not desired in rapid deployments. Further, large setup times hinder operations which require the radio system to be portable or on-the-move.
Referring to graph 19 in FIG. 1, the probability area for the large dipole antenna is represented by the area within dotted and dashed line 25. The probability area for the slant wire antenna is represented by the area enclosed by dotted line 27. The probability for whip antennas is represented by the area surrounded by solid line 29. As can be shown in FIG. 1, the larger dipole antennas have a significantly higher probability of properly receiving and transmitting messages followed in order of highest probability by slant wire antennas and whip antennas, respectively.
As shown in graph 19, the most portable antenna, the whip antenna, has the poorest probability of complete transmissions and receptions (the area enclosed by solid line 29). This factor is especially disadvantageous because the whip antenna is a more practicable antenna for utilization on vehicles or other mobile platforms. Transmission and reception in ranges from 10 to 60 miles is especially poor with the whip antenna. Takeoff angles for smaller antennas, such as, the whip antenna do not have the vertical takeoff angles associated with dipole antennas and therefore have reduced gain.
Therefore, there is a need for an HF NVIS system which can be available for rapid deployments. Further, there is a need for an HF NVIS radio that can be operated on the move. Further still, there is a need for an HF NVIS radio system which has increased transmission and reception performance and can utilize whip antennas. Yet further, there is a need for a method of operating a radio system that improves reception and transmission and can utilize whip antennas. Yet even further, there is a need for a terrestrial communication system for providing for stable communications across a range from zero to 100 miles and yet not require large setup times or significant infrastructure. The approach given below can be used to provide a virtual antenna to other beyond line-of-sight radio propagation paths, such as “regular” HF skywave, meteor burst, or troposcatter systems.